The present invention is directed generally to cardiac pacers and more particularly to a voltage and temperature stable low-power clock circuit for use with implantable battery operated cardiac pacers.
In order to avoid the need for premature invasive battery replacement surgery, circuits intended for use in battery operated implantable cardiac pacers should preferably require minimal electrical power. Furthermore, such circuits should maintain desired operating characteristics despite changes in battery voltage and temperature. Specifically, in the case of pacer clock circuits, the clock frequency should be independent of operating temperature, in order that it can be accurately set while the device is at room temperature (23.degree. C.) prior to implantation within a patient's body.
Clock circuits currently in use with battery operated implantable cardiac pacers are typically of the regenerative RC oscillator type, wherein a capacitor alternately charged and discharged through a resistor, determines clock frequency. Since the frequency determining components in such clock circuits are temperature sensitive, a change in ambient temperature alters the clock frequency unless suitable temperature compensation circuitry is provided. Furthermore, in such regenerative RC circuits, battery current is drawn during both the capacitor charge and discharge cycles, thereby imposing a relatively high power requirement.
Complementary-MOS (CMOS) circuitry is well suited for use in battery operated implantable cardiac pacers since such devices by their nature draw extremely little current while in operation. Consequently, a clock circuit intended for use in an implantable cardiac pacer can employ CMOS active devices to advantage to replace other more power consuming active devices. Since CMOS devices are relatively unaffected by variations in supply voltage, their use in battery-operated circuitry is particularly attractive where it is desired that circuit performance be maintained as the battery ages.
Accordingly, it is a general object of the present invention to provide a new and improved clock circuit for use in battery operated devices such as cardiac pacers.
It is a more specific object of the invention to provide a cardiac pacer clock circuit requiring minimal current during operation.
It is a more specific object of the invention to provide a clock circuit for use in cardiac pacers wherein the clock frequency remains constant over a range of ambient temperatures and supply voltages.